An ultrastructural study of cytodifferentiation in the developing human root-tip.

Transmission electron microscopy showed that in growing human teeth, the root sheath consisted of inner and outer epithelial cells. The inner epithelial cells formed a basal lamina associated filamentous layer which increased in density in coronal direction. Extensions of developing odontoblasts were in contact with the basal lamina. Direct contact with the epithelial plasmalemma was not observed. The odontoblasts obtained their fully-developed cylindrical appearance after making contact with the basal lamina of the inner epithelial cells. But, they possessed already abundant RER before these contacts were present, indicating that the differentiation of dental papillary cells into collagen-producing cells did not require heterotypic epithelio-mesenchymal contacts. After deposition of dentine, the odontoblast processes were withdrawn from the epithelium and the outer layer of root dentine. Differentiation of cementoblasts was observed in the dental follicle in which mesenchymal cells develop into RER-containing cells, migrating through apical root-sheath fenestrations to their final position between the continuous root sheath and outer dentinal layer. The findings suggest that all differentiation steps of odontogenic epithelium and odontogenic papillary and follicular mesenchyme proceed without actual contact between epithelium and mesenchyme.